I think you and I are musical soulmates or something -- or at least we're the only ones obsessive enough to care

Besides, I need to know where you're finding some of these people. I used to use SongJapan.com to find new artists, but they've been effectively shut down for song listening purposes I've had to pass up a couple good sales because I couldn't find out what the artists sounded like before I bought; and I'm not buying music blind.

About the artists you mentioned specifically:

Hip Hop
AI {love her, but she needs to have the right songwriter, or she sounds like absolute crap}
Bennie K [I like that Coke commercial song she did, but otherwise, I haven't really seen much of her}
Heartsdales [love them, but not really sure how I feel about the latest album yet -- need to hear it a few more times]
Crystal Kay [she has a very strong allergy affected voice in many of her recordings that drives me absolutely crazy, but, it can be counteract it with the right headphone sound really good]
Amuro Namie [quite possibly one of the absolute best produced in her genre. Unfortunately, she has no stamina during live concerts. This situation was so bad when they were filming her so crazy tour, that they actually drugged her with some kind of amphetamine with visible results onstage -- my money is on Dexedrine. I remember it not only from my friends who used to have ADD, but for myself when I used to take it. Not only does she have the look of someone on Dexedrine, she even has the voice problems that go along with the right after they dosed her. ]
Miliya Kato [never heard of this person, so you've got to help me find her music}
Ketsumeishi {also never heard of a need to find more on}
R&B
DJ Masterkey [Weird]
Dragon Ash [can't stand them. ]
M-Flo [his good albums start with Astro romantic. Expo Expo is absolutely horriblep without a single redeeming quality and the other album I heard from back when Lisa was part of the group was terribly uneven]
Double [I only like her voice on the more reason stuff. I remember I used to hear her on the radio and she would get on my nerves because her voice was too high. Her guest appearances are also pretty good on other people's albums}
KREVA [yet another person you're going to have to introduce me to]

Rap
ZEEBRA [love his flow and he has an incredible voice]
DELI [don't really care one way or another]

Rock
D.A.I [one of my favorite bands that suffers horribly with the wrong headphones/speakers.]
Orange Range [mostly hate them, but occasionally they'll release something almost tolerable]
The Pillows [love their earlier stuff, but wasn't as impressed with penalty live CD]

Pop
Ayumi Hamasaki [I used to believe that too, but then she grew on me. I now own all her original albums, a couple of remixes, a video collection, and a whopping seven concertsDVDs]
BoA [absolutely love her in Japanese, don't really like her in Korean, and think she mumbles too much in Chinese. Also, her English quality varies from song to song]
Koda Kumi [incredible deep voice {for a Japanese female singer } when produced properly and still very good even when she's not,]
Mika Nakashima [great voice with terrible background music often enough.] ,
Matsuura Aya [I know the name, but can't place the voice]
Emyli [same here, although I think I heard her with m-flo]
Melody [love her latest single, and most of her guest appearances on other people stuff, but I remember not liking her so much on the radio way back]
Hikaru Utada [she should have good English, she's from New York. Been bicoastal most of her life, but her first album was in English -- and flopped. That's when she started recording in Japanese and became successful]
Younha [don't recognize the name, but with your help, I will soon]

now let me add to the list

Amano Tsukiko {melodious female rocker}

anthem {although this band is highly rated by most metal sites that mentioned them in the U.S., aside from some occasional short bursts of guitar brilliance, I think they're overrated}

Ayaka Hirahara {ethereal vocal qualities that can make even a bad song sound good}

Aikawa Ananese {another great female rocker that's also great live -- the sounds like a different person live.}

Aiko {capable of being good, but rarely happens. Latest album is better than I expected to be}

bloodiest saxophone {a good jazz instrumental band that conveys a sense of movement in its music. Unfortunately, they broke up}

denki groove {quirky Techno that's a lot of fun , if not very substantive. Their songs are either very good or very bad with no middleground}

Dorothy { a Korean band that crossed over into Japanese -- and sounds much better in Korea unfortunately}

dream {the original three member dream was very good, but then they became a morning masume clone with none of the talent, with the goal of "graduating" members and bringing in new ones on a regular basis. To make things more problematic, the new dream has remade all of the old dreams songs, say you have to be very careful when you're buying for a specific song}

every little thing {love them most of the time, although I couldn't listen to them every day comfortably}

globe {their level 4 album is nearly flawless, and their older work gets worse the farther you go back. Keep in mind know, most people hold the exact opposite opinion if they were fans of the band in the beginning}

Gackt {a musical god with incredible compositional and vocal skills. Unfortunately, he is so sick now, he's painful to watch live. He's even switched to singing in a less stressful nasal voice that sounds absolutely awful. Granted, he denies being sick beyond fatigue, but you would have to be blind not to see it -- even if he wasn't frequently been hospitalized -- which he is}

hiro {a former member of speed who sometimes sounds like Ayumi's early work vocally. Beginning to develop a new vocal sound though from what I've heard on the radio lately. Just as nice as her old sound, yet more distinctive}

Hello! Project {morning masume and its various spinoffs/contemporaries from the same label release under this name. The live concerts are fun to watch on DVD, but I can't recommend anyone for albums other than the original morning masume}

hitomi {very addictive unique pop sound which hasn't appreciably changed -- but it doesn't matter, because she's consistent in delivering a good time}

move {perfect band {dance music} that changes their sound every album and are absolutely awesome live}

Shunichi Miyamoto {balladeer with a beautiful voice, but is overproduced into oblivion. I had to opportunity to see video of him in a production session and the producer was telling him all the wrong things to do take after take.}

Megumi Hayashibara {an anime voice actress with one good quasi-mainstream song from blue seed. The rest of her stuff is lackluster to crap unfortunately}

Maki Oguru {and early version of koda kumi's deeper voice, with none of her production values. With the exception of her Utada duet, not really that good}

Nana Kitade {another anime voice actress that's tried to crossover to the mainstream {or at least, some sellers are trying to portray that way} very inconsistent from one song to another}

Onmyoza {one of the most perfect rock bands if not the most currently on the scene Also incredibly good live}

porno graffiti {I find myself more addicted to them as time goes on -- very good}

sugar soul {only have one or two good songs}

Shiina Ringo {a singing and compositional goddess -- not to mention a kick ass producer. Of course, everyone she produces ends up sounding like her; but when someone is as consistently good as she is, that's a good thing. She exudes a strong sense of class during live performances unlike anything I seen before with modern performers. However, if you're buying her DVDs, you should know she gets very creative with the live arrangements and if you don't understand her lyrics, you may not recognize your favorite songs}

soulhead {okay for an R&B group on some songs, but just barely}

Tomomi Kahala {listening to her sing is like watching one of your buddies get drunk and then give a concert. Yes, she sounds very drunk and often off key in her old recordings. She recently released the new album called naked, and from what I hear of it, she finally learned to sing quite well. The bars loss is the world's gain I guess

Tackey & Tsubasa {now I love them on their albums. Remember I say this with love: they are the absolute GAYEST performers I've ever seen live. The concert starts of this morning {yes, I watched them for the first time this morning}, by the announcer saying "children can be sexy". Holy crap! This can't possibly get any worse...... and then it does. Not only do they have very gay looking attire, but they even have the limp wrist movements down like the biggest flames can possibly imagine. The whole thing goes downhill from their in terms of credibility as legitimate performers. However, this is the funniest I've ever seen on any concert, and I highly recommend everyone see it at least once just for that reason alone. Although they botch their Inuyasha hit, the rest of the concert is musically good {not including the disco remixes -- did I mention they made their disco songs even more discoLMAO!}

taiyo & Giscomoon {a hello project spinoff whose first album was so bad, they had to change their name to release the second album. Unfortunately, I heard the second album's single and bought the first album by accident -- go head and laugh

UA {a mix of progressive jazz with African / Brazilian sensibility and Japanese lyrics. She absolutely perfect, and the only musical artist I know that can get away with using barking dogs as musical instruments}

zone {a perfect bubble gum pop band that plays their own instruments -- and just broke up unfortunately}

zz {Has the ccasionally really good song, but mostly crap}

aco {pretty voice that needs a really good producer -- it doesn't happen to often}

polysics {extraordinarily popular for some reason among people I talk to, but I've never liked them much}

^ not everybody I own, but most of them

As long as I bothered to post all that, anyone else have some musical recommendations to add?

Wow, thats alot. Well you added some I left out, but whatever. I also left out KinKi Kids, Chemistry, Ryohei Yamamoto, YUI, Sowelu, Mr. Children and Hinoi Team.

KinKi Kids [I don't really listen to them, but I see their music videos doing well on the charts. They have interesting music videos, but like I said, I just tune the music out. I think the thing that interested me was their name, but then I got it, and it was boring.]

Chemistry [Not sure about them, it sounds like they edit their voices alot]

I agree with MenomaMinx on L'Arc~en~ciel. I loved their albums (huge fan here) but when I saw their lives...hrm...has loads of room for improvement. XD Anyway, so far I like New World, Jiyu e no shoutai (can't really remember the title...think the romanji is written as such) from the AWAKE album and Hello from HYDE's 666 album.

I think AliceNine is pretty good as well. And Miyavi (love his voice...so sexy!).

Rie fu is quite good as well. I thought that her voice was really soothing. haha. ^^

just a note... BoA is a Korean singer tho isn't she? Though she is still nice to listen to. I saw her when searching for 'Boa' (ye just the last letter not caps). That was a bit of a pain cuz I couldn't download much to listen. I got some in the end and bought albums. Ah that's English music anyway. If you have ever watched 'Lain' the opening scene is by Boa tho.

To me, Japan has some of the best and most avant-garde in the world. Japan has given birth to some great acts like Sigh, Sabbat, and 五人一首. Since everyone here is talking about j-pop and the such, I don't think any of you guys would have much in interest in these bands. The Japanese have this certain way of taking styles of music and playing around with them until the sound is completely crazy. That's why I love japanese music so much.

As far as more popular music goes, I guess X-japan are pretty good, well at least some of their songs. My main problem with mainstream japanese is that it always seems like they're just picking up on the American trend from a few years ago (the current rap trend is a good example).